1. Field of the Invention
The present invention relates to a monitor for diffusable chemical substances and in particular to a monitor which may be used to analyze an animal (including human) body fluid to determine the presence of diffusable chemical substances.
2. Description of the Prior Art
In order to provide an optimal treatment for a patient, for instance during intensive care, it is important to collect information on the condition of that patient on a regular basis. Important information can be obtained by analyzing the blood of the patient to determine the presence and amount of diffusable chemical substances such as blood gasses, electrolytes, metabolites (such as glucose or urea) or H.sup.+ ions.
It is also important that a monitor used for this type of regular analysis take away as little of the body fluid (such as blood) from the patient as possible. Otherwise the cumulative effect of many individual determinations could itself be detrimental to the patient.
A monitor for the in vivo determination of the presence of diffusable chemicals within a body fluid in which none of the fluid is withdrawn from the patient is described in U.S. Pat. No. 4,221,567. The monitor has a liquid filled probe with a permeable membrane which, when inserted into the body fluid, allows the chemical substances of interest to pass between the body fluid and the liquid. Sensors sensitive sensitive to the chemical substance of interest are placed along the flow path of the liquid; in the probe. A pump transports liquid from the region of the membrane for analysis by the sensors after equilibrium is reached among the diffusable chemicals in the body fluid and in the liquid.
Since the sample volume of liquid that contains the diffused chemicals is relatively small compared to the volume of liquid between the sensors and the membrane in this known monitor, the pump must operate accurately in order to ensure that liquid from the correct region of the probe is analyzed by the sensors. Moreover, the pump must transport the liquid in a smooth yet quick manner in order to avoid dispersion of the chemical substances from the equilibrated volume. All of this requires a relatively expensive pumping and control system.